Animal models are widely used as screening tools for evaluating compounds or treatments during drug or cosmetic development. The available in vitro methods for screening compounds principally involve the use of cell monolayers, co-cultures or isolated skin explants. In vivo methods include the mouse ear swelling test (Gad et al. (1986). Toxicol Appl Pharmacol 84:93–114; Descotes (1988) J Toxicol-Cutan Ocular Toxicol 7:263–272), and the murine local lymph node assay (LLNA) (Kimber et al. (1989) Contact Dermat 21:215–220; Dearman et al. (1999) J Appl Toxicol 19(5):299–306). Transgenic mice models have also been used as screening tools for testing the effect of compounds applied to the skin on the expression of a transgene (Berstein et al., U.S. Pat. No. 5,648,061; Burgeson et al., PCT International Publication No. WO 99/30743). These models require the use of multiple animals and multiple experiments for thorough assessment and controls in screening assays.